1. Field of the Invention
Method and circuit arrangement for printing a print image.
The present invention relates to a method for printing a print image on an endless stock in relation to a predetermined position in an electrographic printer.
2. Description of the Related Art
Electrographic printers are known in which a motor drives a transport means that transports the stock past a transfer printing station, essentially according to a predetermined print speed. During the printing process, print images produced by a printing control unit are printed successively on the stock at the transfer printing station with the print speed. If the transport means communicates a forward motion to the stock by means of positive locking, as is for example the case given the engagement of transport pins in transport holes of a stock perforated in the edge regions, then within determinate limits a constrained movement is realized between the stock and the transport means.
If, however, the transport means communicates the forward motion to the stock by means of non-positive locking, in that a rubberized transport roller or a transport strip stands in non-positive contact with the stock, a constrained movement between the stock and the transport roller or, respectively, transport strip is prevented by various influencing factors. These influencing factors include the microslippage that occurs in that the non-positive connection between the drive roller and the stock so that alignment is not one hundred percent guaranteed. Another influencing factor is to be found in the mechanical tolerances in the manufacture of the drive roller and its mounting in the transport means. A constrained movement between the stock and the drive roller thus does not take place. Since the transport speed of the stock thus never agrees entirely with the speed of the drive roller, a synchronous operation between the stock and the print process is excluded. If, for example, pre-printed form material, applied using offset printing, is present on the stock, in which numbers or letters are supposed to be printed in predetermined form fields during the print process, an offset, due to the minimal speed difference, between the letters and the form fields becomes larger with each printed form. An interruption of the print process in order to correct the offset is unavoidable.
However, the cited problems with respect to the offset also occur when the transport means communicates the forward motion in positively locked fashion to the stock. In this case, a clock generator is fastened to a photoconductor drum in the transfer printing station, which clock generator, via a control circuit for the phase (PLL--phase locked loop) generates the frequency of a stepped motor for driving the transport means, and for an illumination row for the illumination of the photoconductor drum. A disadvantage of this solution is that mechanical stiffnesses, network frequency oscillations, etc., are not completely corrected. Moreover, more complex electronic components are required for the realization of the phase controlling, which mostly operate according to an analog design, i.e. process continuous voltage values.
The object of the invention is to indicate a simple digital solution for printing endless stock that enables an offset-free printing in relation to a predetermined position.
This object is solved by means of a method having the features of patent claim 1. The invention proceeds from the assumption that given essentially constant transport speed of the stock, the offset becomes cumulatively larger from print image to print image. Accordingly, in order to correct the offset, only minimal speed modifications of the transport speed of the stock are necessary. The transport speed can accordingly be used as a reference quantity for a measurement of the offset, despite slight fluctuations.
In the invention, markings are present on the stock at regular intervals, for the positioning of the print images in the direction of transport. At the beginning of the printing of a print image, a print image beginning signal is produced by the print control unit. Given offset-free printing, the markings must always be at the same locations in relation to the transfer printing station when the print image beginning signal occurs, due to the equal spacing and the essentially constant transport speed. An offset expresses itself in that the markings change slowly from print image to print image in relation to a selected reference point in the environment of the transfer station, when only momentary registrations are respectively observed at the reference point at the moment of the print image beginning signal.
In the invention, a signal pickup is fastened in stationary fashion relative to the transfer printing station, which pickup produces a marking signal when a marking is detected. Given an offset, markings located before the signal pickup in the direction of transport will move toward the signal pickup or, respectively, move away from it, if successive momentary registrations are observed at times with an active print image beginning signal, according to whether the transport speed of the stock is somewhat greater or somewhat less than the print speed.
Due to the greater or lesser spacing in relation to the signal pickup, the distance of the marking located closest before the signal pickup can be used to measure the offset by measuring the time required for this marking to reach the signal pickup.
For this reason, when the print image beginning signal occurs, a counting process of clock signals is started in a counter. The counting process is interrupted upon the occurrence of the next marking signal at the signal pickup. The count result stands in a ratio that is determined by the clock signals to the time required for the next marking to reach the signal pickup. From this time, by multiplication by the print speed the distance of the marking from the signal pickup at the time of the print image beginning signal can be calculated. In particular, given a change in this distance from print image to print image, an offset can be recognized. In the invention, the counter result is compared with a target value that corresponds to a counter state given an offset-free positioning of the print images in relation to the marking. If the count result is greater than the target value, the frequency of a current impulse sequence that controls a stepped motor that drives the transport means is increased. If the count result is smaller than the target value, the frequency of the current impulse sequence is reduced, so that the marking is again removed from the signal pickup upon the next print image beginning signal.
Both the counter and also the stepped motor controlling can be realized digitally. Since a microprocessor is present in the printer, the comparison can also easily be carried out digitally. In the invention, only one counter and an easily modifiable impulse controlling for the stepped motor need be used for the synchronization of the transport speed of the stock and of the print speed.
As markings, e.g. the transport holes of the stock or a cross-perforation, present if necessary at the beginning and at the end of a print page, can be used, so that no additional expense also arises due to the markings. Given stock that is free of transport holes, an offset-free printing is required only when the stock is already for example printed with pre-printed form material before the printing process. The required additional expense for the application of the markings in offset printing is likewise low, since the markings are applied at the same time as the pre-printed form material.
In an embodiment of the invention, a pulse generator contains a divider and a frequency multiplier, whereby the divider is clocked at the input side with a basic clock of the basic frequency, and emits an output pulse sequence that has a frequency that is defined by the ratio of the basic frequency and a divider value determined by the comparison result. The frequency multiplier multiplies the frequency of the output pulse sequence by a whole-number value and outputs the control pulse sequence. By means of this measure, it is achieved that the positioning precision is increased, since in order to achieve an operating frequency range of the stepped motor that is e.g. co-determined by the transport means, higher divider values are required than without the use of a frequency multiplier. Given higher divider values, smaller frequency changes occur in the predetermined frequency operating range, despite the subsequent frequency multiplication per divider value step, i.e. an increasing or a lowering by one. Smaller frequency changes have the result that the stock is also offset only by small distances, so that a small offset can also be corrected.
In addition, the invention relates to a circuit arrangement for printing a print image on an endless stock in relation to a predetermined position in an electrographic printer, having a pulse generator for producing a pulse sequence of a predetermined frequency according to a predetermined value, having a stepped motor controlled by the pulse sequence of predetermined frequency, which motor drives a transport means that transports the stock past a transfer station, essentially according to a predetermined print speed, having a control unit for producing the print images that are printed successively on the stock at the transfer printing station with the print speed, having a signal pickup that scans the stock for markings that are present at regular intervals on the stock for the positioning of the print images in the direction of transport, whereby the signal pickup produces a marking signal when a marking is acquired, having a counter that starts a count process of clock signals, said process being started by means of a print image beginning signal that is produced by the control unit at the beginning of the printing of a print image, and which interrupts the count process upon the occurrence of the marking signal or after the occurrence of a fixed number of marking signals, whereby the control unit compares the count result with a target value, which corresponds to a counter state given offset-free positioning of the print images in relation to the markings, and whereby the control unit sets a predetermined value in the pulse generator dependent on the result of the comparison. The circuit arrangement serves to carry out the method according to the invention, so that the above-named effects are also transferred to the circuit arrangement.